1. Field of the Invention
The present invention relates generally to a carrier sense technique and system and, more particularly, to communications technology used in the Class B Carrier Sense Automatic Identification System (AIS), for instance.
2. Description of the Related Art
Beginning in 2002, the Safety of Life at Sea (SOLAS) regulations have required ships of specified classes (hereinafter referred to as SOLAS ships) to be equipped with the Automatic Identification System (AIS) as a tool for streamlining vessel traffic services (VTS) and promoting collision avoidance, for instance.
An AIS unit compulsorily installed on a SOLAS ship broadcasts messages containing such information as the name of ship, position, navigation status and ship movement, dimensions (length and beam) of ship, type of ship, type of cargo, draft and destination and receives the same kinds of information from other AIS-equipped ships near own ship, so that each SOLAS ship can acquire information on other ships in a particular area. The use of the AIS system enables own ship to verify not only the presence of nearby ships but also such information as the name of ship, dimensions (length and beam), type of ship, type of cargo, draft and destinations of each nearby ship which have been impossible or difficult to obtain by radar observation.
The AIS system provides a communications network using phase-modulated signals for which two very high frequency (VHF) channels are assigned. The AIS network employs predefined communications techniques to avoid conflicts (collisions) of radio transmissions. These techniques include so-called “Class A AIS” which is currently used for SOLAS ships and “Class B AIS” which will be implemented in equipment installed on non-SOLAS ships on a non-mandatory basis.
Class A AIS uses Self-Organized Time Division Multiple Access (SOTDMA) technology in which each one-minute period referred to as a “frame” is divided into 2250 time slots each of which is 26.7 ms long. A Class A AIS unit uses each of these time slots as a unit of transmission of the aforementioned information.
One characteristic feature of Class A is that each Class A AIS unit simultaneously transmits own ship information and slot reservation information as mentioned in Japanese Patent Application Publication No. 1995-501879. For the sake of explanation, it is now assumed that there are three AIS-participating ships, designated Ship A, Ship B and Ship C, and a Ship A station, for instance, transmits own ship information together with slot reservation information specifying a slot in a next frame to be reserved for its own use. In this situation, a Ship B station transmits its own ship information together with slot reservation information specifying a slot in the next frame to be reserved while avoiding a slot allocation conflict with the Ship A station. A Ship C station then transmits its own ship information and slot reservation information while avoiding slot allocation conflicts with the Ship A and Ship B stations. The AIS-participating ships repetitively carry out such a sequence from one frame to the next while communicating respective transmission schedules with due attention to avoid multi-user scheduling conflicts.
For successful SOTDMA communication of this kind, it is essential that all participating stations be precisely synchronized using a common time slot scheduling scheme. AIS equipment described in the aforementioned Publication has a built-in Global Navigation Satellite System (GNSS) receiver, such as a Global Positioning System (GPS) receiver, which provides an accurate one pulse-per-second (1 PPS) signal obtained from a received satellite signal. All AIS stations are precisely synchronized by using this 1 PPS signal.
On the other hand, Class B AIS equipment is a low-cost AIS unit designed for installation on a non-SOLAS ship. As Class B AIS units normally use Carrier Sense Time Division Multiple Access (CSTDMA), Class B AIS is hereinafter referred to as Class B (CS) AIS, “CS” representing Carrier Sense technology.
To avoid any conflicts with transmission schedules of Class A AIS stations installed on SOLAS ships, each Class B (CS) AIS station monitors transmissions from Class A stations to see if there is other traffic before initiating a message transmission. The Class B (CS) station transmits own message in a specific time slot only after verifying the absence of other radio traffic (i.e., signals carrying information transmitted from any Class A stations).
To be more specific, the Class B (CS) station performs the following procedure when transmitting a message:    (1) Find a Class A station synchronized with GPS time by receiving signals from other AIS stations in an asynchronous fashion;    (2) Determine timing kept by a time base of the GPS-synchronized Class A station from bit timing of that station;    (3) Using the timing of the Class A station time base, choose 10 future time slots (hereinafter referred to as “candidate slots”) in which the Class B (CS) station may carry out “carrier sense” operation to detect a carrier of an AIS information signal;    (4) Sense (or “listen to”) a carrier of any AIS signal for approximately 2 ms at the start of a candidate slot in which own station wants to transmit;    (5) If the candidate slot is judged to be free with no information signals from other AIS stations detected therein as a result of the carrier sense operation, immediately transmit own message in the pertinent slot; and    (6) If the candidate slot is judged to be occupied with an information signal from another AIS station detected therein as a result of the carrier sense operation, abandon the pertinent slot, wait and retry transmission in a next candidate slot.
Class B (CS) AIS allows a short duration of time of approximately 2 ms at the start of a slot for performing the carrier sense operation, so that each Class B (CS) station must judge whether there exists a signal transmitted from a Class A station during approximately the 2 ms time duration. It is however extremely difficult for a Class B (CS) station to achieve such a high receiver gain that is high enough to permit a reliable judgment on the presence or absence of an AIS signal within such a short time duration (approximately 2 ms), compared to a case where the Class B (CS) station is allowed a time duration of approximately 26.7 ms to receive a complete time slot and make a judgment based thereupon. Although it would be possible to increase the receiver gain using a narrow-band filter design, a group delay caused by the narrow-band filter design results in a reduction in the number of bits that can be processed within a given period of time.
In addition, a typical method of the carrier sense operation based on measurement of the intensity of a received radio signal can cause a misjudgment in carrier sensing in the presence of high-level background noise or erratic noise and, therefore, the 2 ms time duration is too short to make a reliable judgment on the presence or absence of an AIS signal.